Gas chromatography and mass spectrometry are being applied to the analysis of nucleosides in hydrolysates of transfer RNA following conversion to trimethylsilyl derivatives. Quantitative measurements of the antitumor drug ftorafur and its metabolites in the rodent are being carried out. Several nucleosides have been labeled with deuterium and a detailed analysis of their fragmentation patterns will be carried out. The structure of new modified nucleosides from transfer RNA is being studied by mass spectrometry.